The present invention relates to a deglitch circuit for removing noise.
Conventionally, a regulator output is sometimes superimposed with noise, or a so-called glitch. For example, as shown in FIG. 9, when a comparator is used to determine whether a regulator output has an “H” or “L” level with a reference voltage VSCP, the output may be superimposed with a glitch that is greater than the reference voltage VSCP. In such a case, a deglitch circuit is used as a filter circuit for removing the glitch. A Schmitt trigger circuit may be used as a deglitch circuit for removing a glitch (for example, Japanese Laid-Open Patent Publication No. 8-237087). The configuration of this deglitch circuit will be described with reference to FIGS. 10A and 10B. As shown in FIG. 10A, the deglitch circuit includes an inverter, an RC filter, and a Schmitt trigger circuit. The RC filter functions as a lowpass filter to remove noise of a predetermined frequency or greater. The Schmitt trigger circuit is typically formed by a circuit that is shown in FIG. 10B. Such Schmitt trigger circuit has hysteresis characteristics as shown in FIG. 10C.
If noise occurs in a power supply line of the Schmitt trigger circuit, as shown in FIG. 10D, the voltage VOUT of the output signal of the deglitch circuit will be repeatedly shifted between “H” and “L” levels in the vicinity of the threshold of the Schmitt trigger circuit. In other words, noise cannot be removed when power supply noise is large. As is obvious from the hysteresis characteristics in FIG. 10C, power consumption is also increased if noise occurs on the power supply line in the vicinity of the threshold of the Schmitt trigger circuit.
Japanese Laid-Open Patent Publication No. 7-336201 describes an improved configuration of the deglitch circuit as shown in FIGS. 10A to 10D. This configuration will be described with reference to FIGS. 11A and 11B. The deglitch circuit as shown in FIG. 11A further includes a NAND circuit. Once the output signal of this circuit is shifted between “H” and “L” levels, the voltage VC is reduced to the ground level. Therefore, even if noise occurs on the power supply line in the vicinity of the threshold of the Schmitt trigger circuit, the output signal voltage VOUT of the deglitch circuit will not be repeatedly changed between “H” and “L” levels.
However, the deglitch circuit shown in FIG. 11A is also formed by a Schmitt trigger circuit. Thus, power supply current will continue to be flow in the hysteresis range. This will increase power consumption. In particular, when change in the voltage VC is gradual, the power consumption becomes outstanding. Additionally, the time constant of the RC filter must be determined in accordance with the noise situation. However, if the time constant is once determined, it cannot be changed. This makes is difficult to flexibly cope with various situations. When the voltage VC changes, current will flow in the vicinity of the threshold voltage as shown in FIG. 12, and power will be consumed. Therefore, if the voltage gradually changes as in the deglitch circuit shown in FIGS. 10 and 11, the power consumption will increase.